A retrovirus designated human immunodeficiency virus (HIV) is the etiological agent of the complex disease that includes progressive destruction of the immune system (acquired immune deficiency syndrome, hereinafter AIDS) and degeneration of the central and peripheral nervous system. There are two types of HIV, HIV-1 and HIV-2, the latter producing a less severe disease than the former. Being a retrovirus, its genetic material is in the form of RNA (ribonucleic acid) consisting of two single RNA strands. Coexisting with RNA are reverse transcriptase (having polymerase and ribonuclease activity), integrase, a protease and other proteins.
It is known in the art that some antiviral compounds which act as inhibitors of HIV replication are effective agents in the treatment of AIDS and similar diseases. Drugs that are known and approved for the treatment of HIV-infected patients belong to one of the following classes:                nucleoside reverse transcriptase inhibitors (NRTI) such as, but not limited to, azidothymidine, zidovudine, lamivudine, didanosine, abacavir, adefovir and the like,        nucleotide reverse transcriptase inhibitors (NtRTI) such as, but not limited to, tenofovir (commercially available under the trade name Viread),        non-nucleoside reverse transcriptase inhibitors such as, but not limited to, nevirapine, efavirenz and the like,        protease inhibitors such as, but not limited to, nelfinavir, saquinavir, ritonavir, indinavir, amprenavir, fosamprenavir and the like, and        fusion inhibitors such as enfuvirtide.        
A relatively new target that was focused on lately is the integrase enzyme of HIV, while also many other proteins acting as enzymes or co-factors are being investigated.
Replication of the human immunodeficiency virus type 1 (hereinafter referred as HIV-1) can be drastically reduced in infected patients by combining potent antiviral drugs targeted at multiple viral targets, as reviewed by Vandamme et al. in Antiviral Chem. Chemother. (1998) 9:187-203.
Multiple-drug combination regimes can reduce viral load below the detection limit of the most sensitive tests. Nevertheless low level ongoing replication has been shown to occur, possibly in sanctuary sites, leading to the emergence of drug-resistant strains, according to Perelson et al. in Nature (1997) 387:123-124.
Furthermore the selectivity of many antiviral agents is rather low, possibly making them responsible for side-effects and toxicity. Moreover, HIV can develop resistance to most, if not all, currently approved antiviral drugs, according to Schmit et al. in J. Infect. Dis. (1996) 174:962-968. It is well documented that the ability of HIV to rapidly evolve drug resistance, together with toxicity problems resulting from known drugs, requires the development of additional classes of antiviral drugs.
As a summary, there is still a stringent need in the art for potent inhibitors of HIV. Therefore a goal of the present invention is to satisfy this urgent need by identifying efficient pharmaceutically active ingredients that are less toxic and/or more resistant to virus mutations than existing antiviral drugs and that can be useful, either alone or in combination with other active ingredients, for the treatment of retroviral infections, in particular lentiviral infections, and more particularly HIV infections, in mammals and more specifically in humans. Furthermore, another goal of the present invention is complement existing antiviral drugs in such a way that the resulting drug combination has improved activity or improved resistance to virus mutation than each of the individual compounds.
The family of the Flaviviridae consists of 3 genera, the pestiviruses, the flaviviruses and the hepaciviruses and also contains the hepatitis G virus (HGV/GBV-C) that has not yet been assigned to a genus. Pestiviruses such as the Classical Swine Fever Virus (CSFV), the Bovine Viral Diarrhea Virus (BVDV) and the Border Disease Virus (BDV) cause infections of domestic livestock (respectively pigs, cattle and sheep) and are responsible for significant economic losses world-wide. Vaccines are used in some countries with varying degrees of success to control pestivirus disease. In other countries, animal culling and slaughter are used to contain pestivirus disease outbreaks.
The World Health Organization estimates that world-wide 170 million people (3% of the world's population) are chronically infected with HCV. These chronic carriers are at risk of developing cirrhosis and/or liver cancer. In studies with a 10 to 20 year follow-up, cirrhosis developed in 20-30% of the patients, 1 to 5% of whom may develop liver cancer during the next then years. The only treatment option available today is the use of interferon α-2 (or its pegylated from) either alone or combined with ribavirin. However, sustained response is only observed in about 40% of the patients and treatment is associated with serious adverse effects. There is thus an urgent need for potent and selective inhibitors of the replication of the HCV in order to treat infections with HCV. Furthermore, the study of specific inhibitors of HCV replication has been hampered by the fact that it is not possible to propagate HCV (efficiently) in cell culture. Since HCV and pestiviruses belong to the same virus family and share many similarities (organisation of the genome, analogous gene products and replication cycle), pestiviruses have been adopted as a model and surrogate for HCV. For example BVDV is closely related to hepatitis C virus (HCV) and used as a surrogate virus in drug development for HCV infection.
In view of their important pharmacological value, there is a need for drugs having antiviral activity against viruses belonging to the family of Flaviviridae including hepatitis C virus.
Pioneering work on the chemistry of phosphonate nucleosides has already been carried out and includes certain important reaction schemes to synthesize phosphonate nucleosides. A review of chemistry and biology of phosphorous-modified nucleotide analogues is available for instance from A. Holy in Advances in Antiviral Drug Design (1993) 1:179-231. Phosphonate nucleosides can be devided in two categories. A first category are real nucleoside analogues since they contain a nucleobase and a sugar moiety. A second category of phosphonate nucleosides, represented for instance by 9-(2-phosphonyl-methoxyethyl)adenine (adefovir), can be considered as alkylated nucleobases since their sugar moiety is replaced by an alkoxyalkyl moiety. Surprisingly, up to now, potent antiviral in vivo activity (HSV, CMV, HBV, HIV) has only been associated with certain phosphonalkoxyalkyl nucleobases and not with sugar containing phosphonate nucleosides. Several attempts to discover antiviral nucleoside phosphonates have led to synthetic schemes for the preparation of furanose-, pyranose- and carbocyclic phosphonate nucleosides, all of them however lacking potent antiviral activity.
Phosphorylation by kinases and incorporation into nucleic acids (eventually leading to chain termination) is considered as an important mechanism which may explain the antiviral activity of nucleosides. The lack of antiviral activity of phosphonate nucleosides of the first category is generally explained by their poor substrate properties for cellular and viral kinases. On the other hand, the potent antiviral activity of phosphonylated alkylated nucleobases of the second category has been ascribed to their intracellular phosphorylation into diphosphates and to an incorporation of the modified nucleosides into nucleic acids (enzymatic incorporation into nucleic acids being almost irreversible) which has negative consequences downstream and thereby inhibits viral growth. A disadvantage of the acyclic nucleoside phosphonates are their low selectivity index in cellular screening systems. The selectivity of the triphosphates of anti-HIV nucleosides for HIV reverse transcriptase versus mitochondrial DNA polymerases is usually regarded as an important factor determining in vivo toxicity. Thus there is still a need in the art for drug candidates having suitable selectivity for HIV reverse transcriptase. A less flexible structure such as is present in the nucleosides phosphonates is considered to improve both binding to polymerases and viral-versus-host selectivity. Consequently, nucleosides phosphonates retaining their HIV reverse transcriptase affinity are considered as strong antiviral candidates.
Threose nucleosides have been previously synthesized because they can be assembled from natural precursor molecules. It has been demonstrated that threose nucleic acids (TNA) form duplexes with DNA and RNA exhibiting a thermal stability similar to that of the natural nucleic acids association. Triphosphates of threose nucleosides are accepted as substrate by several polymerases and can be enzymatically incorporated in DNA. A few 2,5-dihydro-5-(phosphonomethoxy)-2-furanyl nucleosides derived from thymine and adenine with antiretroviral activity have been disclosed by Kim et al. in J. Org. Chem. (1991) 56:2642-2647. EP-A-398,231 describes a family of phosphonomethoxy-methoxymethyl purine/pyrimidine derivatives being effective in combating viral infections at a dose of 0.01 to 30 mg/kg bodyweight. U.S. Patent Publication No. 2004/0023921 discloses a pharmaceutical composition comprising a nucleotide analog with a phosphonate group at an amount effective to inhibit a viral polymerase of an hepatitis C virus (hereinafter referred as HCV) or to act as a substrate for the viral polymerase of the HCV virus. WO 98/20017 describes a family of modified nucleoside-5′-triphosphates which are inhibitors or substrates of DNA polymerases and antiviral agents, being in particular able to inhibit the reproduction of the human HIV virus in a culture of human lymphocytes.
Although, as is apparent from the prior art of record, numerous compounds were proposed for meeting the various above mentioned requirements in terms of retroviral therapy, it was observed that none of them does achieve such goals and, consequently, there is still a stringent need in the art for new compounds being able to solve these problems.